1. Field of the Invention
The present invention relates to a patch measurement device, and more particularly to a patch measurement device, which can be incorporated in a printing apparatus, for measuring the color density of patches constituting a control strip which is printed on printing paper.
2. Related Art Statement
There have conventionally been realized printing apparatuses which incorporate a so-called CTP (Computer To Plate) device, i.e., a prepressing device (=a printing plate recording device) that generates an image on a printing plate based on “image-to-print data”, i.e., data representing an image to be printed. A printing apparatus of this type, referred to as a digital printing press, is capable of producing printed materials directly from image data, and therefore may be suitable for producing a variety of printed materials, each in a relatively few copies, over short periods of time. While prepress and other processes in such a printing apparatus are automated for ease of operation by operators with insufficient proficiency, further automation is desired in the adjustment of the amounts of inks and/or dampening water to be supplied during a printing process.
The control of ink supply in a conventional printing apparatus is generally realized by means of a separate console-type color measurement device, where a produced sample print is measured on a table. In this case, there is a problem in that a human operator needs to take out sample prints from the printing apparatus as necessary to measure the colors appearing on the printed materials.
A printing apparatus which realizes automatic control of an ink supply amount, etc., is disclosed in Japanese Patent No. 2824334, for example. Hereinafter, the disclosed apparatus will be referred to as a “first conventional printing apparatus. The first conventional printing apparatus previously retains reference image data representing a printed material that serves as a reference against which to adjust the ink supply amount. Moreover, after producing a printed material on an internal impression cylinder, the first conventional printing apparatus generates “printed-image data”, i.e., data representing the actually produced printed material. Furthermore, the first conventional printing apparatus compares the generated printed-image data and the reference image data to determine whether to increase or decrease the supply amounts of inks and/or dampening water. Based on the determination result, the first conventional printing apparatus automatically adjusts the ink supply amounts. Thus, the first conventional printing apparatus has an advantage in that, since printed-image data is generated within the printing apparatus, there is no need to bother an operator as in the case of employing a console-type color measurement device.
However, the printed-image data which is generated from an actually printed material tends to have a relatively large data size, and so does the reference image data. Therefore, the first conventional printing apparatus has a problem in that the comparison between the reference image data and the printed-image data consumes substantial time. Another problem is the need to prepare the reference image data in advance. In these respects, the first conventional printing apparatus is not suitable for producing relatively few copies of a variety of printed materials, where agility is of the essence.
Therefore, the Applicant has previously filed an application directed to a second conventional printing apparatus which realizes prompt adjustment of the ink supply amounts by printing not only an image of a subject of printing but also control strips (each comprising a number of patches) and reference marks on a printed material, and further generating printed-image data, which represents the control strips and the reference mark, by means of an internal imaging device. The second conventional printing apparatus further comprises a patch measurement device for measuring color density information of the printed patches on the basis of the printed-image data. The color density information may include, for example, the density and/or dot percentage of the printed patches. Thus, the second conventional printing apparatus compares the color density information obtained by means of the patch measurement device against a previously set reference value to determine whether to increase or decrease the supply amount of ink or dampening water. Based on this determination result, the second conventional printing apparatus adjusts the supply amount of ink or the like.
Now, the specific operation of the aforementioned patch measurement device will be described. FIG. 16A is a diagram illustrating a printed material S which may be obtained by using the second conventional printing apparatus. As shown in FIG. 16A, the second conventional printing apparatus prints an image im on printing paper, and thereafter prints four control strips cs1 to cs4 and three reference marks rm1 to rm3 on the same printing paper. Hereinafter, such four control strips cs1 to cs4 may collectively be referred to as “control strips cs”, and the three reference marks rm1 to rm3 as “reference marks rm”.
The image im is printed on the printing paper, beginning at a position (hereinafter referred to as a “print start position”) which is located a predetermined gripper margin f away from the leading end of the printing paper. More specifically, the image im is progressively printed in the direction of print progress indicated by the arrow (hereinafter referred to as a “first printing direction”), beginning from the print start position. The image im has a dimension m along the first printing direction, which is designated according to the image size. The control strips cs and the reference marks rm are printed beginning at a position which is a predetermined distance n away from the trailing end of the image im.
As shown in FIG. 16A, the control strips cs are typically printed on the printing paper with predetermined intervals therebetween along a direction (hereinafter referred to as a “second printing direction”) perpendicular to the first printing direction, and each control strip cs includes a plurality of rectangular-shaped patches arranged in a predetermined order. Each patch may be a half-tone, linework, or solid image which is printed at a predetermined density in a predetermined color. FIG. 16B illustrates an exemplary patch pc1.
As shown in FIG. 16A, the reference mark rm1 is interposed between two adjoining control strips cs2 and cs3. The reference mark rm2 is interposed between the control strips cs1 and cs2, and the reference mark rm3 is interposed between the control strips cs3 and cs4. As such, the reference marks rm1 to rm3 serve as references based on which to detect the positions of the control strips cs1 to cs4. Typically, as exemplified by the reference mark rm1 shown in FIG. 16B, a reference mark comprises two bars b1 and b2 which run parallel to the first printing direction, and a cross mark c interposed between the bars b1 and b2. Each patch is printed at a position which is predetermined distances away—along the first and second printing directions—from a crosspoint p of the cross mark c. For example, the patch pc1 is printed so that the center thereof is at a distance a (along the first printing direction) and at a distance b (along the second printing direction) from the crosspoint p of the reference mark rm1.
The printed-image data representing the printed material S is generated by capturing an image of the printed material S within the printing apparatus, and passed to the patch measurement device. Assuming that the patch pc1 is currently to be processed by the patch measurement device, the patch measurement device first detects the crosspoint p of the reference mark rm1. Furthermore, the patch measurement device estimates that a position which is at the distance a (along the first printing direction) and at the distance b (along the second printing direction) from the detected crosspoint p should be the center position of the patch pc1, which is currently to be processed. Thereafter, the patch measurement device measures the at the color density information of the patch pc1 at the estimated position.
On the other hand, each reference mark rm, which is printed in a single color of B (black), is a mark used for positioning purposes. On the immediately upper side of a region in which the control strips cs are printed (closer to where the image im is printed) is a predetermined blank region which is purposely left white, i.e., no images are printed. It is ensured that the bars b1 and b2 are longer than the width (along the first printing direction) of the region in which the control strips cs are printed, and long enough to encompass part of the blank region. Accordingly, any region in which a detectable portion of the bars b1 and b2 appears along the first printing direction can be determined as part of the region in which the control strips cs are printed, and/or part of the blank region.
In order to detect a reference mark rm from the printed-image data, the second conventional printing apparatus employs pattern recognition technique. The method for detecting a reference mark rm begins by previously obtaining a pixel pattern of the neighborhood of the center of the cross mark c interposed between the bars b1 and b2 in the reference mark rm. FIG. 17 illustrates an exemplary pixel pattern of an α region, which is in the neighborhood of the center of the cross mark c shown in FIG. 16B. For ease of understanding, the illustrated example is made purposely schematic. A center line β of the cross mark c consists of: two black pixels on each side (constituting the width of each of the bars b1 and b2); a white pixel interposed between a horizontal stroke of the cross mark c and each of the bars b1 and b2; and seven black pixels composing the entire horizontal stroke of the cross mark c. Next, it is determined whether or not the pixel pattern of the center line β shown in FIG. 17 is contained in the captured printed-image data while shifting the examined pixels one by one. This technique is applied with respect to each of the X and Y directions (which are perpendicular to each other) on a memory storing the printed-image data.
Correlation coefficients are employed in the calculations use for matching the pixel pattern against the printed-image data. For example, the pixel pattern of the above-described center line β can be expressed in a binary representation “1101111111011”. Correlation coefficients ρ are sequentially calculated with respect to a key pattern x (i. e., the line β) and subject data y (i.e., data to be matched against the key pattern x), while shifting the subject data y by one pixel. Specifically, the line β used as the key pattern x is:x=(1,1,0,1,1,1,1,1,1,1,0,1,1).The subject data y is:y=(y−6,y−5,y−4,y−3,y−2,y−1,y0,y+1, y+2,y+3,y+4,y+5,y+6)The correlation coefficient ρ is expressed as:ρ=([x]*[y])/(σx×σy).
Note, however, that the term [x]*[y] in the above equation is defined as a sum of the multiplication products of corresponding elements of the respective matrices, as opposed to a mathematical product (|x|*|y|) of the two matrices in the traditional sense. Specifically, if each matrix consists of one row×three columns, then(u1v1w1)*(u2v2w2)=(u1×u2)+(v1×v2)+(w1×w2)under the above definition. Note that σ x is a standard deviation of the key pattern x, and σ y is a standard deviation of the subject data y. The calculated correlation coefficients are compared, and the position associated with the highest correlation coefficient is regarded as the position of the reference mark rm.
Next, the problems associated with the above-described patch measurement device will be described. As mentioned above, each control strip cs includes a plurality of patches which are arranged along the second printing direction. In the case where there are fifteen ink keys in the printing apparatus, the total number of patches would be 60 or more. However, due to limited spaces being available for printing the patches pc and the reference marks rm, the total number of reference marks rm which are printed on the printing paper is disproportionately small relative to the large number of patches. Even if an increased number of reference marks rm is employed, it would only invite an increase in the detection frequency of the reference marks rm, thereby resulting in more time being consumed for measuring the color density information. In this respect, the total number of reference marks rm should be minimized. However, employing a small number of reference marks rm has a disadvantage in that the prescribed distance from each reference mark rm to each patch becomes more prone to error as the number of reference marks rm is decreased. Specifically, a greater error is expected for patches which are disposed father away from the reference mark rm. The effect of such errors is that the patch measurement device may erroneously measure the color density information related to positions not corresponding to the patch centers. Thus, inaccurate color density information may be obtained.
Moreover, the above-described printing apparatus is constructed so that printed-image data is generated as the printed material S is read by an internal imaging device. However, since the printed material S is read during its transportation, the read position of the printed material S may fluctuate. Moreover, due to recoil and like actions of the printed material S during its transportation, pixels which normally compose a rectangular-shaped patch may present a parallelogram or diamond-shaped congregation in the printed-image data. In that case, even if the distances a and b (for the first and second printing directions, respectively) are added to the coordinates of the crosspoint p which is detected from the printed-image data, the result may not indicate the proper center of the patch pc1. In this respect, too, the conventional patch measurement device may not be able to measure accurate color density information.
Furthermore, since the calculation of correlation coefficients used in the detection of the reference marks rm involves division by the standard deviations of the key pattern x and the subject data y, any pattern which happens to resemble the key pattern x may produce a large correlation coefficient ρ, irrespective of the data sizes (signal intensities) of the respective pixels, thus falsely indicating a strong correlation. In other words, if a pattern resembling the key pattern x happens to be present in the neighborhood of a reference mark rm due to flares in the optical system, print smears, and the like, that pattern may erroneously be recognized as a reference mark rm, however weak the signal levels of such pixels may be. This will hinder the detection of the actual reference mark rm, and an incorrect position may instead be detected. Consequently, since the position of the reference mark rm is not properly detected, it becomes impossible to detect the patches pc composing the associated control strips cs, so that the color density information of the patches pc cannot be measured.